1. Field of the Invention
The present invention relates generally to nuclear reactors and, more particularly, to an improved grid and resulting fuel assembly for a pressurized water nuclear reactor.
2. Description of the Related Art
In most pressurized water nuclear reactors, the reactor core is comprised of a large number of elongated fuel assemblies. These fuel assemblies typically include a plurality of fuel rods held in an organized array by a plurality of grids that are spaced axially along the fuel assembly length and are attached to a plurality of elongated thimble tubes of the fuel assembly. The thimble tubes typically receive control rods or instrumentation therein. Top and bottom nozzles on opposite ends of the fuel assembly are secured to the ends of the thimble tubes that extend slightly above and below the ends of the fuel rods.
The grids, as is known in the relevant art, are used to precisely maintain the spacing between the fuel rods in the reactor core, resist rod vibration, provide lateral support for the fuel rods and, to some extent, vertically restrain the rods against longitudinal movement. One type of conventional grid design includes a plurality of interleaved straps that together form an egg-crate configuration having a plurality of roughly square cells which individually accept the fuel rods therein. Depending upon the configuration of the thimble tubes, the thimble tubes can either be received in cells that are sized the same as those that receive the fuel rods therein, or can be received in relatively larger thimble cells defined in the interleaved straps.
The straps of previously known grids have been configured such that the cells each include a pair of relatively compliant springs and four or more relatively rigid dimples, with the springs and dimples being formed into the metal of the interleaved straps and protruding outwardly therefrom. The springs and dimples of each cell engage the respective fuel rod or thimble tube extending through the cell. Outer straps of the grid are attached together and peripherally enclose the inner straps of the grid to impart strength and rigidity to the grid.
Depending upon the specific configuration of the grid and the straps, the straps may each include one or more mixing vanes formed thereon that facilitate mixing of the water within the reactor and thus promote convective heat exchange between the fuel rods and the water. Since the reactor is designed such that the water moves generally vertically upward through each fuel assembly, the mixing vanes, while promoting mixing of the water, nevertheless impart forces and torques to the grid as a result of the water impinging on the vanes. Such forces and torques on the grid can result in stresses and vibrations being applied by the grid to the fuel rods.
It is also known in the relevant art that during operation of the reactor the elevated temperatures, pressures, and fluid velocities within the reactor tend to cause vibrations between the grids and the fuel rods. Since the grids support the fuel rods within the fuel cell, such vibrations therebetween can result in fretting of the fuel rods. Such fretting, if sufficiently severe, can result in corruption of the fuel rod with resultant nuclear contamination of the water within the reactor. It is thus desired to provide an improved grid having springs and dimples that are designed to minimize fretting wear between the grids and the fuel rods.
As is known in the relevant art, the straps of such grids typically are made of known materials such as Zircaloy or other such materials which are suited to the nuclear environment within a nuclear reactor. It is known, however, that Zircaloy tends to grow after prolonged periods of exposure to a nuclear environment, and such growth is even greater in regions of the straps that are work hardened due to metal forming operations. Such growth hampers the dimensional stability of the fuel assembly, and it is thus desired that any such improved grid likewise promote and maintain the dimensional stability of the fuel assembly. It is further desired that such an improved grid have enhanced thermal-hydraulic performance and reduced fabrication costs.
In view of the foregoing, an improved grid for use in a fuel assembly of a nuclear reactor includes a plurality of straps that are interconnected with one another in a lattice fashion to define a plurality of cells, with the straps being configured to include a plurality of relatively more compliant springs and a plurality of relatively less compliant dimples, whereby each cell includes only one pair of springs and one pair of dimples disposed therein. The springs and dimples are contoured to engage a fuel rod or a thimble tube disposed within the cell. Each spring directly confronts one of the dimples to provide confronting pairs of features that engage the fuel rods or thimble tubes, with each cell including two pairs of confronting features. The ligaments of the springs and dimples extend in a direction generally perpendicular to the direction of coolant flow through the reactor in order to minimize pressure drop.
Accordingly, an aspect of the present invention is to provide an improved grid for a fuel assembly of a nuclear reactor in which the grid is configured to reduce fretting wear between the grid and the fuel rods.
Another aspect of the present invention is to provide such a grid having springs and dimples that are formed to include a contour that corresponds with the fuel rods.
Another aspect of the present invention is to provide such a grid having springs and dimples that are configured to reduce the pressure drop of coolant passing through the nuclear reactor.
Another aspect of the present invention is to provide such a grid having straps that are formed to include a plurality of springs and dimples, with the springs and dimples each being of substantially the same height and width, whereby each spring directly confronts only a single dimple to provide pairs of directly confronting support features for supporting fuel rods within cells of the grid.
Another aspect of the present invention is to provide such a grid that provides improved thermal-hydraulic performance.
Another aspect of the present invention is to provide such a grid that can be fabricated at a reduced cost.
Another aspect of the present invention is to provide such a grid that promotes dimensional stability of a fuel assembly into which the grid is incorporated.
Another aspect of the present invention is to provide an improved fuel assembly that incorporates an improved grid incorporating one or more of the features set forth above.
Accordingly, an aspect of the present invention is to provide a grid for use in a fuel assembly of a nuclear reactor, the grid being structured to carry a plurality of cylindrical members including at least one of a fuel rod and a thimble tube, each of the cylindrical members including an outer surface, in which the general nature of the grid can be stated as including a plurality of straps, the straps being interconnected with one another in a lattice fashion to define a plurality of cells, each strap including a plurality of strap members, each strap member defining a wall of at least one of the cells, each strap member including a protruding spring and a protruding dimple, the spring including a spring plate formed to include a spring contour, and the dimple including a dimple plate formed to include a dimple contour, the spring contour and the dimple contour each being structured to substantially correspond with and engage at least a portion of the outer surface of an associated cylindrical member, and a pair of the springs and a pair of the dimples extending into each of the cells, one of the pair of the springs and one of the pair of the dimples directly confronting one another, the other of the pair of the springs and the other of the pair of the dimples directly confronting one another.
Another aspect of the present invention is to provide a grid for use in a fuel assembly of a nuclear reactor, the grid being structured to carry a plurality of cylindrical members including at least one of a fuel rod and a thimble tube, each of the cylindrical members including an outer surface, in which the general nature of the grid can be stated as including a plurality of straps, the straps being interconnected with one another in a lattice fashion to define a plurality of cells, each strap including a strap axis and a plurality of strap members, the strap axes together lying generally within an imaginary grid plane, each strap member including a frame, a spring, and a dimple, the spring and the dimple protruding from the frame, the spring including a spring plate and a pair of spring ligaments, and the dimple including a dimple plate and a pair of dimple ligaments, the spring being structured to engage at least a portion of the outer surface of an associated cylindrical member, the dimple being structured to engage at least a portion of the outer surface of an associated cylindrical member, each spring ligament including a spring ligament axis extending between the frame and the spring plate, and each dimple ligament including a dimple ligament axis extending between the frame and the dimple plate, the spring ligament axes and the dimple ligament axes all being oriented generally parallel with the grid plane, and a pair of the springs and a pair of the dimples extending into each of the cells, one of the pair of the springs and one of the pair of the dimples directly confronting one another, the other of the pair of the springs and the other of the pair of the dimples directly confronting one another.
Another aspect of the present invention is to provide a fuel assembly for use in a nuclear reactor, the general nature of which can be stated as including a plurality of cylindrical members including at least one of a fuel rod and a thimble tube, and at least a first grid, the at least first grid including a plurality of straps interconnected with one another in a lattice fashion to define a plurality of cells, the cylindrical members being disposed in the cells, each strap including a plurality of strap members, each strap member including a protruding spring and a protruding dimple, the spring including a spring plate and a pair of spring ligaments, and the dimple including a dimple plate and a pair of dimple ligaments, the spring ligaments and the dimple ligaments all extending in a direction generally transverse to the longitudinal extent of the cylindrical members, and wherein a pair of the springs and a pair of the dimples extend into each of the cells, one of the pair of the springs and one of the pair of the dimples directly confronting one another, the other of the pair of the springs and the other of the pair of the dimples directly confronting one another.
Another aspect of the present invention is to provide a grid for use in a fuel assembly of a nuclear reactor, the grid being structured to carry a plurality of cylindrical members including at least one of a fuel rod and a thimble tube, in which the general nature of the grid can be stated as including a plurality of straps, the straps being interconnected with one another in a lattice fashion to define a plurality of cells, each strap including an upstream edge, each strap including a downstream edge, the upstream edge and the downstream edge being disposed generally opposite one another, each strap including a plurality of strap members, each strap member defining a wall of at least one of the cells, each strap member including a protruding spring, each strap member including a protruding dimple, the spring and the dimple being disposed at alternate sides of at least a portion of a single groove formed in the associated strap member, the groove being disposed generally midway between the upstream and downstream edges.